Effects of artificial weathering in NR/SBR elastomer blends
Valentin Cepus et. al.
Abstract
Degradation of polymer blends occurs by the constituent phases undergoing distinct chemical changes that depend on their unique chemical structures. This makes predicting and establishing a structure-property relationship for each phase necessary as well as challenging. In this work, the molecular and physical changes occurring in sulfur-cross-linked natural rubber (NR), styrene?butadiene rubber (SBR), and their 50/50 blend subjected to accelerated weathering are analyzed by ?H nuclear magnetic resonance (NMR) spectroscopy, Fourier-transform infrared (FTIR) spectroscopy, atomic-force microscopy (AFM), and dynamic mechanical thermal analysis (DMTA). NMR transverse relaxation time (T2) studies suggest the formation of rigid components due to weathering. FTIR and AFM reveal that this is related to the formation of a stiff surface due to chemical modifications, which shows up as an additional thermal transition in the DMTA curves. Low-field double-quantum (DQ) NMR studies of the cross-link density, by the residual dipolar coupling constant (Dres), of SBR show a continuous increase in its cross-link density over the weathering duration (988 h). In contrast, NR exhibits dominant chain scission reactions resulting in defects, with both materials demonstrating the formation of different chain lengths. During the first 168 h, NR also undergoes modification of sulfur bond lengths, which is also observed in the blend. The blend largely follows an intermediate trend of cross-link densities compared to the two polymers but shows signs of lesser chain modifications than a weighted average of the two p?o?l?y?...